Twice John identifies Jesus as the "Lamb/Ἀμνὸς of God" (ESV):
The next day he saw Jesus coming toward him, and said, “Behold, the Lamb (Ἀμνὸς) of God, who takes away the sin of the world! (John 1:29)
and he looked at Jesus as he walked by and said, “Behold, the Lamb (Ἀμνὸς) of God!” (John 1:36)
In Revelation, "The Lamb" is always called Ἀρνίον and is never called Ἀμνὸς. For example:
saying with a loud voice, “Worthy is the Lamb (Ἀρνίον) who was slain, to receive power and wealth and wisdom and might and honor and glory and blessing!” (Revelation 5:12)
Ἀμνὸς and Ἀρνίον are different words identified as synonyms yet do not have an etymological connection. According to the King James concordance Ἀρνίον is used only in Revelation and in the plural form in John 21:15:
When they had finished breakfast, Jesus said to Simon Peter, “Simon, son of John, do you love me more than these?” He said to him, “Yes, Lord; you know that I love you.” He said to him, “Feed my lambs (ἀρνία).” (John 21:15)
Peter is to feed the plural lambs/ἀρνία of Jesus. Since the singular ἀρνίον is the diminutive of ἀρήν, Peter's feeding of the diminutive ἀρνία lambs, implies there is a greater Lamb and logically Jesus is the Ἀρήν.
John's Gospel presents two possible words whose meaning is Lamb to refer to Jesus either "Lamb/Ἀρήν" or "Lamb/Ἀμνὸς." In addition, since (plural) lambs ἀρνία are to be fed by Peter, so too the (singular) lamb ἀρνίον is to be fed by Peter.
In considering the language of the Gospel and Revelation, I see two questions:
- Why call the Lamb of Revelation "Ἀρνίον" not "Ἀμνὸς?" Why not call the Lamb who was slain The Lamb/Ἀμνὸς of God who takes away the sin of the world?
- Why call the Lamb in Revelation by the diminutive "Ἀρνίον" not "Ἀρήν"?